In the design of automobile alternators, several factors must be considered. In general, there must be a compromise between acceptable speeds for the alternator and the desired output. Tests show that the alternator output current drops drastically when the alternator speed falls below 3,000 rpm. On the other hand, the alternator output current rises only slightly when the alternator speed exceeds 10,000 rpm. Under ideal conditions, the alternator would operate at a continual speed change dependent on the required alternator output. Present alternator operation is accomplished by standard fixed belt pulley ratios from the crankshaft of the engine to the alternator input pulley, thus alternator speed changes with engine speed. This ratio is determined by the required minimum alternator output current and the minimum engine idle speed. While this method may work well at lower engine speeds, higher engine speeds cause excessive alternator speeds; consequently, alternator design becomes very difficult. Predicted future trends show that alternator loads will increase dramatically and, in some instances, this is already a problem.
Possible solutions to this problem are:
(1) Increase the pulley ratio. Although this would work well for lower speeds, high engine speeds would cause alternator overspeed.
(2) Increase the engine idle speed. This would adversely affect the vehicle's drivability and fuel economy.
(3) Add another alternator. This solution would be costly, would have a negative impact on the engine's performance, and would be very difficult to package.
(4) Develop a multi-speed alternator drive unit. Constant speed drives tend to be bulky and expensive, however, a two-speed drive unit could be developed that would be cost effective and easily packaged.
The present invention relates to a two-speed alternator drive unit to address this problem.